1. Field of the Invention:
The present invention relates to a semiconductor laser, and more particularly to a III-V group compound semiconductor laser.
2. Description of the Related Art:
FIG. 8 shows a cross-sectional view of an AlGaAs/GaAs laser as an example of a conventional III-V group compound semiconductor laser (see FIG. 1 of J.J. Coleman et al., Applied Physics Letters, Vol. 37, No. 3, pp. 262-263, (1980)).
In FIG. 8, A1 is an n-GaAs substrate, A2 an n-Al.sub.x Ga.sub.1-x As (x=0.35) lower cladding layer, A3 a GaAs active layer, A4 a p-Al.sub.x Ga.sub.1-x As (x=0.35) first upper cladding layer, A5 an n-GaAs layer for confining current and light with respect to the horizontal direction (hereinafter, referred to as "current and light confining layer"), A6 a p-Al.sub.x Ga.sub.1-x As (x=0.35) second upper cladding layer, and A7 a p-GaAs contact layer.
Hereinafter, a method for fabricating the above-mentioned semiconductor laser will be described.
First, the lower cladding layer A2, the active layer A3, the first upper cladding layer A4, and the current and light confining layer A5 are successively grown on the substrate A1. Then, a channel (strip groove) with a width of 4 .mu.m is formed in the current and light confining layer A5 by etching. After this, the second upper cladding layer A6 and the contact layer A7 are successively grown on the resulting current and light confining layer A5. All of the crystal growth is conducted by the Metal Organic Chemical Vapor Deposition ( MOCVD ) method.
Next, the operation of the above-mentioned semiconductor laser will be described.
When a voltage is applied between the contact layer A7 and the substrate A1, a forward current flows within the stripe groove of the current and light confining layer A5, through the forward biased pn junction between the first upper cladding layer A4 and the active layer A3. A region including the current and light confining layer A5 outside the stripe groove has pnpn-junctions, so that a reverse bias is applied between the current and light confining layer A5 and the first upper cladding layer A4, resulting in no current flow therebetween. Thus, the current is confined within the stripe groove. In addition, since the layers including the current and light confining layer A5 outside the stripe groove have a smaller equivalent refractive index than that of the layers within the stripe groove because of the light absorption by the layers outside the stripe groove, a laser beam is optically confined within the stripe groove.
FIG. 9 shows another conventional example of a semiconductor laser (see S. Yamamoto et al., Applied Physics Letters, Vol. 40, No. 5, pp. 372-374, (1982)).
In FIG. 9, B1 is a p-GaAs substrate, B2 an n-GaAs current and light confining layer, B3 a p-Al.sub.x Ga.sub.1-x As (x=0.4) lower cladding layer, B4 an Al.sub.x Ga.sub.1-x As (x=0.13) active layer, B5 a p-Al.sub.x Ga.sub.1-x As (x=0.4) upper cladding layer, and B6 a p-GaAs contact layer.
A method for fabricating the above-mentioned semiconductor laser will be described.
First, the current and light confining layer B2 is grown on the substrate B1, and after that a channel (stripe groove) with a width of 4 .mu.m is formed in the current and light confining layer B2 by etching until it reaches the surface region of the substrate B1. Then, the lower cladding layer B3, the active layer B4, the upper cladding layer B5, and the contact layer B6 are successively grown on the resulting current and light confining layer B2. The crystal growth in this conventional example is conducted by the Liquid Phase Epitaxy (LPE) method. The current and light confining structure of this laser is substantially the same as that of FIG. 8.
In the case of the above-mentioned two conventional examples, an n-GaAs single layer is used as the current and light confining layer. Hereinafter, a conventional example in which a multi-layer is used as the current and light confining layer (see Japanese Laid-Open Patent Publication No. 1-304793) is shown in FIG. 10. In FIG. 10, C1 is an n-GaAs substrate, C2 an n-AlGaAs lower cladding layer, C3 an active layer, C4 a p-AlGaAs upper cladding layer, C5 an N-GaAs melt-back layer, C6 an n-Al.sub.x Ga.sub.1-x As (x=0.4) etching stop layer, C7 an n-GaAs current blocking layer, C8 an n-Al.sub.x Ga.sub.1-x As (x=0.4) anti-melt-back layer, C9 an n-GaAs melt-back layer, C10 a p-Al(GaAs cap layer, and C11 a p-GaAs contact layer. A method for fabricating this type of semiconductor laser is almost the same as that of FIG. 8 except that selective etching is used for the purpose of improving controllability of a stripe depth. Because of this, the etching stop layer C6 is provided. In addition, when a stripe groove is formed by etching, the melt-back layer C5 is retained, and the melt-back layer C5 is removed by melting back at the time of the second growth. Thus, the melt-back layer C5 prevents a regrowth interface (i.e., the surface of the upper cladding layer C4 within the stripe groove) from deteriorating at the time of the second growth. The second growth is conducted by the LPE method. The anti-melt-back layer C8 prevents the current blocking layer C7 from being melted back, and the melt-back layer C9 facilitates the crystal regrowth above the anti-melt-back layer C8.
Next, another conventional example in which a multi-layer is used as the current and light confining layer is shown in FIG. 11 (see H. Ishikawa et al., Applied Physics Letters, Vol. 36, No. 7, pp. 520-522, (1980)). In FIG. 11, D1 is an n-GaAs substrate, D2 a p-Al.sub.x Ga.sub.1-x xAs (x=0.32, thickness d.sub.D2 =0.7 .mu.m ) current and light confining layer, D3 an n-GaAs (thickness d.sub.D3 =0.1 to 0.2 .mu.m) current and light confining layer, D4 an n-Al.sub.x Ga.sub.1-x As (x=0.32) cladding layer, D5 a p-Al.sub.x Ga.sub.1-x As (x=0.05) active layer, D6 a p-Al.sub.x Ga.sub.1-x As (x=0.32) cladding layer, and D7 a p-GaAs contact layer. A method for producing this type of semiconductor laser is almost the same as that of FIG. 10 except that the current and light confining layer is formed of two layers.
Conventional semiconductor lasers have the structures shown in FIGS. 8 to 11 and have been put into practice. However, the inventors extensively studied the above-mentioned semiconductor lasers; as a result, they found the following four critical problems.
(Problem 1) PA0 (Problem 2) PA0 (Problem 3) PA0 (Problem 4)
In the conventional examples shown in FIGS. 8 and 9, a current and light confining mechanism depends on a pn reverse bias between the n-GaAs current and light confining layer and the cladding layer excluding the stripe groove channel. FIG. 4 shows a band diagram of the pnpn-junctions of the semiconductor laser shown in FIG. 8 at the thermal equilibrium state. The semiconductor laser shown in FIG. 8 is made of layers whose material and mixed crystal composition are different from those of the semiconductor laser shown in FIG. 9. However, the relative size of each band gap in the respective semiconductor lasers is the same. Because of the requirements for the light confining layer described later, the band gap E.sub.g2 of the current and light confining layer is made sufficiently smaller than the band gap E.sub.g1 of the active layer. Since holes in the p-type cladding layer do not flow due to a valence band barrier of the n-GaAs current and light confining layer against thereto, a current is confined in the current and light confining layer. In these conventional examples, the current and light confining layer is made of n-GaAs and has a small band gap (i.e., E.sub.g2 =1.42 eV). Thus, the height of the barrier is not large. In addition, because of the relationship E.sub.g1 .gtoreq.E.sub.g2, laser light generated in the active layer is absorbed in the current and light confining layer. Electrons and holes are generated as a result of the absorption of the laser beam, and the holes (minority carriers) diffuse and disappear from the current and light confining layer, whereby only the electrons remain in this layer. When the current and light confining layer is turned on by a phototransistor effect, a current and light confining function is completely lost. In order to overcome this drawback, in the conventional examples, it is required that the thickness of the current and light confining layer is made larger than a diffusion length of the minority carriers. Because of this, even if the current and light confining layer is made of n-GaAs so as to make holes whose diffusion length is short (approximately 1 .mu.m) the minority carriers, the thickness of the current and light confining layer should be about 1 .mu.m. On the other hand, when the current and light confining layer is made of p-GaAs, the thickness of the current and light confining layer should be made even thicker because of a long diffusion length of the electrons (approximately 2 .mu.m).
As described above, it was found that there had been tremendous restrictions on the design of a layer structure. Therefore, in the case where the current and light confining layer is as thick as 1 .mu.m, and a stripe groove is formed through this layer, an etching time becomes long and the amount of etched side walls of the stripe groove becomes large, resulting in the stripe with a width larger than the desired one.
The optical confining of a laser beam in a transverse mode in the above-mentioned four conventional examples is conducted by the absorption of the laser beam in the n-GaAs current and light confining layer. More particularly, there is the following relationship among a real part .epsilon..sub.r of a complex dielectric constant .epsilon., a refractive index n, and an attenuation coefficient k of each layer: EQU .epsilon..sub.r /.epsilon..sub.o =n.sup.2 -k.sup.2
A refractive index with respect to a laser beam in a certain mode (i.e., an equivalent refractive index) depends on a complex dielectric constant of each layer. According to the above equation, in the current and light confining layer, .epsilon..sub.r becomes small, so that an equivalent refractive index becomes small. Because of this, the equivalent refractive index outside the stripe groove is smaller than that within of the stripe groove; as a result, a laser beam in a transverse mode is optically confined within the stripe groove. However, in the conventional examples in FIGS. 8 and 9, the current and light confining layer should be as thick as 1 .mu.m for the reason described in Problem 1. Therefore, the laser beam in the optically confined transverse mode will be excessively absorbed by the current and light confining layer.
Moreover, in FIG. 10, due to the presence of the etching stop layer C6, the current blocking layer C7 could have been made thin. However, since this problem was not taken into consideration until it was pointed out in the present specification, the thickness d of the current blocking layer C7 was set in the range of 0.3.ltoreq.d.ltoreq.2 .mu.m. As described later, unless the thickness of the current blocking layer C7 is made equal to or less than half of a reciprocal of an absorption coefficient (.alpha.) with respect to the laser lasing wavelength of this layer, i.e., 1/2 (0.25 .mu.m at .lambda.=780 nm), the optically confined laser beam in a transverse mode is excessively absorbed. Thus, in the semiconductor laser in FIG. 10, the laser beam in a transverse mode is excessively absorbed in a similar way to the conventional examples in FIGS. 8 and 9.
As described above, it was found that the absorption loss of the laser beam in a transverse mode in the conventional examples becomes excessive. If the loss by absorption is great, an external differential quantum efficiency is decreased, so that a driving current is increased at the time of a high output operation, reducing the reliability of the semiconductor laser. Thus, it was found that it is effective for the improvement of reliability of the high output operation, when the absorption loss can appropriately be reduced.
In the conventional example of FIG. 10, the control of the equivalent refractive index within and outside the stripe groove is not taken into consideration. Because of this, a laser beam in an anti-guide mode, in which the equivalent refractive index outside the stripe groove is larger than that within the stripe groove, is strongly coupled to a laser beam within the stripe groove. As a result, the semiconductor laser cannot function in a normal manner. More specifically, the conventional example in FIG. 10 has a problem that the difference in equivalent refractive index within the stripe groove from that outside the stripe groove cannot be controlled.
In the conventional example in FIG. 11, the current and light confining layer D3 and the cladding layer D4 have the same conductivity type, so that there is no effect of current and light confining. Thus, the diffusion of the current up to the active layer D5 is large. In addition, even if the current and light confining layer D3 had a conductivity type different from that of the cladding layer D4, since the current and light confining layer D3 is thin (i.e., d.sub.D3 =0.1 to 0.2 .mu.m), the current and light confining layer D3 is turned on due to a phototransistor effect and there is no effect of current and light confining. Furthermore, the difference in equivalent refractive index within the stripe groove from that outside the stripe groove and the absorption loss, which determine behavior of the laser beam in a transverse mode, are determined only by the thickness of the current and light confining layer D3 and that of the cladding layer D4, as described in detail in the cited reference. Thus, the desired difference in equivalent refractive index cannot be regulated, independent of the loss by absorption.
As described in Problem 2, the absorption loss of a laser beam in a mode which propagates outside the stripe groove is large, so that a wave front of the laser beam in the mode which propagates outside the stripe groove is remarkably delayed compared with that within the stripe. The delay of a wave front causes an astigmatic difference and has an adverse effect on an image-formation characteristic.